The present invention relates to a method, apparatus and system for forecasting strength of chemically active materials while hardening, and, more particularly, to a method, apparatus and system for non-destructive forecasting of concrete strength.
As used herein throughout the specification and in the claims section below, the phrase “chemically active material(s)” includes cementitious materials, such as, but not limited to, cement paste, mortar, concrete, lime, gypsum, clay and the like that undergo a curing process when hardening.
A chemically active material often needs to be analyzed so as to determine the structural properties parameters, particularly strength and other physical-mechanical properties of the final cured product, such as its potential for shrinkage. The final strength of a chemically active capillary-porous material is determined by the mixing and compacting conditions, and by its specific composition such as, but not limited to, mineral binder-to-aggregate ratio, water-to-cement, water-to-aggregate ratio and the like [Neville A. M., “Properties of concrete,” Longman Scientific & Technical, 1981].
The hardening process of a chemically active material consists of several stages, each characterized by a different combination between the liquid phase and the solid phase of the material.
FIGS. 1a–b show a typical strength curve of a cement of type CEM-1 Portland (42.5N), according to the European Standard EN-197, Part 1: “Composition, specifications and conformity criteria for common cements.” FIGS. 1a–b show the compressive strength, R, of the cement in Megapascals (MPa), as a function of time, t, in a linear (FIG. 1a) and a logarithmic (FIG. 1b) time scale.
Generally, the hardening process is as follows [Powers T. C. and Brownyard T. L., “Studies of the physical properties of hardened Portland cement paste” (9 parts), Journ. Amer. Concr. Inst., 43 (October 1946 to April 1947); Shtakelberg D. I. and Sithcov M. M., “Self-organization in disperse systems,” Riga, “Zinatne” Press, 1990]. Immediately following the mixing and compaction of the cement, the material typically has a long-range coagulation structure. This long-range coagulation structure gradually changes to a short range coagulation structure in which a colloid capillary-porous body is formed. At this stage the liquid phase of the structure is continuous, while the solid phase is discrete. The solid particles present in the material interact through the intermediating liquid and the mechanical stability of the structure is determined by the compressive action of capillary menisci.
Due to the accumulation of reaction products, the concentration of the solid phase increases, disturbing the continuity of the liquid phase, thus forming a structure characterized by a discrete liquid phase and a discrete solid phase. The discretization of the liquid phase is accompanied by an ongoing crystallization processes in which solid-phase contacts appear in the places where the liquid phase is disrupted, first as coagulation type contacts and thereafter as crystalline type contacts. As a result of the formation of crystalline type contacts, the strength of the material is significantly increased.
In the final stage of hardening, the concentration of crystalline type contacts continue to increase until a continuous crystalline frame is formed. The material in this stage is characterized by continues solid phase and a discrete liquid phase.
Traditional prior art methods for testing the strength of concrete typically require 28 days to complete. The builder usually does not or cannot delay construction 28 days to receive the test results. Rather, the construction usually continues in the hope that the concrete is sound. If in the final analysis, the concrete does not meet the standards, the building may have to be reinforced or even torn down, perhaps incurring major additional costs.
Improvements in cement production and prediction of concrete performance properties require the application of material science. Methods of predicting the final strength of concrete while hardening have been developed over the years, based on theoretical and experimental investigations of the crystallization strengthening laws and the properties of chemically active material [to this end see, e.g., Abrams D. A. “Design of concrete mixtures,” Bull. No. 1, Sruct. Mater. Lab., Lewis Inst., Chicago, 1918; Powers T. C., “Structure and physical properties of hardened Portland cement paste,” J. Amer. Ceramic. Soc., 41, 1958, pp. 1–6; Roy D. M. and Gouda G. R., “Porosity-strength relation in cementitious materials with very high strength,” J. Amer. Ceramic. Soc., 53, No. 10, 1973, pp. 549–550; Sheikin A. E., Chekhovsky I. V. and Brusser M. I., “Structure and properties of cementitious concrete,” Stroyizdat Press, Moscow, 1973 (in Russian)].
It has been established that the strength, R, of a chemically active material is determined by the following strength-porosity power-law:R=A*Pm,  (EQ. 1)where P is the porosity of the material, and A and m are constants.
Equation 1 is applicable only within the limited domain of the crystalline state of the material, only after the material experiences the aforementioned structure conditions. In the second hardening stage, the crystalline properties of the body are predominant. Denoting the initial (zero) mechanical strength of the material by Rcr(0), an estimation of the strength of the material 28 days after its mixing can be calculated using Equation 2, below:
                                          R            28                    =                                    R                              cr                ⁡                                  (                  0                  )                                                      ⁢                                          log                ⁢                                                                  ⁢                28                                            log                ⁢                                                                  ⁢                n                                                    ,                            (                  EQ          .                                          ⁢          2                )            where: n denotes the age of the material (in days) at the time where the initial strength Rcr(0) is reached.
One method to predict the concrete strength [King J. W. H., “Further notes on the accelerated test for concrete,” Chartered Civil Engineer, London, May 1957, pp. 15–19] is based just on Equations 1 and 2 above. The essence of the method consists in accelerated determination of the strength of concrete warmed up to 200° F. (93° C.) and aged 6 hours with subsequent extrapolation of the obtained results and strength correlation for the 7-day and 28-day age.
Also known [Y. Ono, “Microscopic observation of clinker for estimation of burning condition, grindability and hydraulic activity,” Proc. 3d Intern. Conf. Cem. Microscopy, Houston, 1981] are attempts to predict the 28-day strength of cement proceeding from the measurement results of Portland cement clinker crystals by means of an electronic microscope, using the following empirical formula:R(kgf/cm2)=253+6.4AS+21.9AB+4.0BS+21.5BC,  (EQ. 3)where AS is the size of alite crystals, AB is the birefringence of alite, BS is the size of belite crystals and BC is the color of belite.
In an additional method [Sinha S. K., Rao L. H. and Akhouri P. H., “Rapid estimation of the 28-day compressive strength of clinker by optical microscopy,” Proc. 13th Intern. Conf. Cem. Microscopy. ICMA, Florida, April 1991], the 28-day strength of cement is determined by the following empirical formula:R(kgf/cm2)=81.6+7.5X1+1.11X2+3.63X3+5.73X4,  (EQ. 4)where X1 is the percentage of the alite, X2 is the percentage of the belite, X3 is the average grain size of alite expressed in percents and X4 is the average grain size of belite expressed in percents.
The above and other prior art methods are expensive and complicated, and require either transportation of a sample to the laboratory or a highly trained material scientist or technician having the proper instrumentation in the construction site.
Since cement stone, concrete and other similar materials at any stage of hardening are poly-dispersed moist capillary-porous bodies, concrete strength can determined by measuring the energy of physically bound water, which is contained in the pores and capillaries of its structure. This energy is indicative of the porosity of the material and therefore of its strength.
Water (both in a liquid and gaseous form) is always in a state of thermodynamic equilibrium with the porous solid phase with which it interacts. Thus, the properties of water (viscosity, bounding energy, relaxation time, etc.) are changing in strict accordance with structure formation and, consequently, with the strength growth of the hardening material. To this end, see, for example, Shtakelberg D., I., supra; Shtakelberg D. I., “Thermodynamics of water-silicate disperse materials structure-formation,” Riga, Zinatne, 1984; and Neville M. “Properties of concrete,” Longman Scientific & Technical. NT., 1988.
In a newly compressed cement paste, whose strength is minimal, e.g., in the order of 10−1 Mpa, practically all the water is distributed between the grains of a non-hydrated cement. The average distance between the grains is approximately 5–10 μm. At this state, the bond energy of water molecules and the material constitutes only a few kDz/mol. While hardening, a portion of the water becomes chemically bound, i.e., transforms into a solid state with bond energy in the order of 1000 kDz/mol. Another portion of the water is contained in the pores of the formed cement gel. The size of these pores is less than 10−3 μm in diameter and the bond energy in this case is up to 50 kDz/mol. Another portion of the water occupies capillaries of a larger diameter (10−2–10−1 μm) with bond energy of up to 10–20 kDz/mol.
Information pertaining to the energy level of water contained in a concrete structure reflects its porosity, which, in turn reflects its strength. Therefore, it is possible to obtain a far more reliable correlation between the energy of water contained in a concrete structure and its strength.
It was already noted above that physically-chemically bound water in capillary-porous bodies always coexists in thermodynamic equilibrium with the solid phase. Nevertheless, all quality changes developing in cement stone and concrete during the process of structure forming and hardening, such as, chemical dispergation, colloidation, coagulation, crystallization, nucleation, development of inner cracks, etc., are almost immediately reflected by the energy of the liquid stage thereof. This is why, namely, the physically bound water is the most informative component of capillary-porous structures for quality evaluation of energetic level and consequently strength and other physical-mechanical properties.
There are various of absorption methods for quantitating (in terms of mass) and qualitating (in terms of energy) chemically and physically bound water in capillary-porous bodies. However, these methods are rather complicated and labor-consuming. Moreover, performance of such measurements in areas of a high relative water vapor pressure is complicated due to development of capillary condensation. In addition, adsorption methods are suitable solely for testing the samples of cement stone, concrete, etc., with a completely-formed or artificially stabilized structure.
U.S. Pat. No. 6,396,265, the contents of which are hereby incorporated by reference, discloses a method of prediction a strength of a chemically active material by performing a high frequency, spin-echo nuclear magnetic resonance (NMR) measurement of the water. More specifically, in the NMR method, a first spin echo NMR measurement is performed at the moment of setting-start of the concrete, and a second spin echo NMR measurement is performed at the moment of setting-finish of the concrete. On the basis of these NMR measurements, a relation of RF-field absorption of energy of physically bound water is determined.
Although the NMR method is significantly more efficient than the absorption methods, this method requires an NMR apparatus to be present at the construction cite. It is appreciated that such apparatus is expensive and is primarily designated for the purpose of laboratory studies.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method, apparatus and system for measuring and forecasting strength of chemically active materials while hardening devoid of the above limitations.